Waste minimization in zeolites synthesis from coal fly ash by recycling alkaline waste

Zeolite A can be synthesized from coal fly ash via a two-step fusion–hydrothermal method, but this process generates alkaline wastewater containing trace heavy metals, hindering large-scale application. This study explores three alkaline wastewater recycling strategies - full recycling, full recycling with reduced hydrothermal time, and partial recycling - to minimize waste and improve process sustainability. Results show that reusing alkaline wastewater can reduce fresh sodium hydroxide input by 20 %, as it supplies the necessary alkalinity for synthesis. However, full recycling leads to the progressive transformation of zeolite A into hydroxyl sodalite, resulting in pure hydroxyl sodalite product when alkalinity exceeds 3.7 mol/L. This effect can be mitigated by reducing hydrothermal time as alkalinity builds up. Overall, full recycling can improve the utilization efficiency of sodium and hydroxide ions over successive batches and reduce wastewater volume and water consumption by 80 % and 62 %, respectively. Among the approaches, partial recycling maintained stable alkalinity and yielded high-purity zeolite A. Additionally, hydroxyl sodalite formation was traced to both the dissolution of fly ash and the transformation of zeolite A itself. These findings highlight a promising strategy for sustainable zeolite synthesis from coal fly ash with significant reductions in chemical and water usage.